1. Field of the Invention
This invention relates to the field of aircraft deicers, specifically to the rapid heating and delivery of deicing fluids, and especially to rapid heating and delivery of high viscosity nonconvective or pseudoplastic deicing fluids.
2. The Prior Art
Definitions
Deicing
Aircraft deicing is the process of removing snow and ice from the wings, tail and other aircraft surfaces, while the plane is on the ground. Deicing is accomplished by spraying hot deicing fluid on the aircraft surfaces.
Anti-Icing
Anti-icing is the proces of preventing ice and/or snow accumulation between the time the aircraft is deiced and the time it takes off. Anti-icing is accomplished by providing a low freezing point deicing fluid on the aircraft surfaces after the deicing/anti-icing operation has been completed.
Deicers
Aircraft deicing and anti-icing are generally accomplished by the use of vehicles, called deicers, which incorporate the following elements: one or more tanks to carry deicing fluids; a means for raising the temperature of the deicing fluid from its storage temperature to the desired spraying temperature; a pump to raise the fluid pressure to a level adequate for spraying; plumbing including hoses and spray nozzles to permit the fluid to be applied to the aircraft; and a boom carrying a basket, or a ladder with a platform at the top to allow the operator to raise the spray nozzle off the ground to a height sufficient for proper application of the deicing fluid.
Deicing Fluids
General
There are three types of deicing fluids in widespread use. Hot water is used by some airlines for deicing (removal of snow and/or ice). It is not suitable for anti-icing. Type I deicing fluid, a mixture of ethylene glycol and water, is the principal deicing fluid used in the United States. Type II deicing fluid (a mixture of propylene glycol, diethylene glycol, a long chain polymer and water) is the principal deicing fluid used in Europe.
Hot Water
Pure hot water is used by some airlines to remove the snow and/or ice accumulations from aircraft. It is readily available, is relatively inexpensive, and does not pose an environmental hazard. Water does not provide any anti-icing protection, and in fact could be the source for ice formation at subfreezing ambient temperatures. It must therefore be followed by the application of a low-freezing-point deicing fluid to provide anti-icing protection.
Type I Deicing Fluid
The Type I deicing fluid which is most widely used in the United States is a mixture of ethylene glycol and water. One commercially available Type I fluid includes 50% water and 50% ethylene glycol. Pure ethylene glycol freezes at 9 degrees F., but the 50/50 mixture with water freezes at -33 degrees F. An alternative 60/40 mixture of ethylene glycol and water freezes below -80 degrees F. Type I fluid provides anti-icing protection, since any snow or freezing rain falling on the aircraft will mix with the residual Type I fluid remaining on the surface to form a non-freezing liquid. Type I fluid has a relatively low viscosity, and therefore flows off the aircraft surfaces quickly. It provides only a short period of deicing protection, on the order of a few minutes in severe weather. Type I deicing fluid is considered to be an environmental hazard, and its use is increasingly being subjected to restrictions in the United States and elsewhere.
Type II Deicing Fluid
Type II fluid is the most widely used deicing fluid in Europe. It is a mixture of propylene glycol and diethylene glycol with water, to which a long chain polymer has been added to provide the desired "pseudo-plastic" viscous properties. The fluid can be sprayed mixed with additional water, or as the "neat" (undiluted) Type II fluid. A typical Type II fluid might have a freezing point of -36 degrees F. The 50/50 mixture of this Type II fluid with water freezes at 14 degrees F. It poses less of an environmental hazard than Type I fluid. The viscosity of Type II fluid is dependent on the velocity at which air flows over the fluid resting on the aircraft surfaces. It has a very high viscosity when the aircraft is stationary or moving at low speed. It therefore stays on the aircraft surfaces after spraying, preventing icing prior to takeoff. The viscosity of the fluid decreases rapidly as the aircraft picks up speed when it starts to move down the runway. Virtually all of the deicing fluid flows off the aircraft before it rotates for takeoff. This avoids a degradation of the aerodynamic characteristics of the aircraft in flight. The viscous behavior of Type II fluid allows it to provide much better anti-icing protection than Type I fluid. Anti-icing protection with a typical Type II fluid lasts for a minimum of twenty minutes, and under some conditions for several hours.
One widely used commercially available Type II fluid contains a small percentage (less than 2%) of a long chain polymer. This polymer provides the fluid with its desirable viscosity behavior. Unfortunately, this long chain polymer can be easily damaged so that the fluid degrades (loses its desired viscous properties). Mechanical degradation can occur under flow conditions which generate shear between adjacent layers of fluid. Thermal degradation can occur by exposure to high temperature surfaces, or by storage of the fluid at elevated temperatures. Mechanical degradation of Type II fluid can occur if the fluid flows through a centrifugal pump because of the turbulence generated by passage of the vanes through the fluid. Degradation also occurs when the fluid flows through a passage at velocities over six feet per second, or passes at comparable velocities over sharpedged surfaces, or undergoes flow separation. Flow turbulence due to high velocities or turbulence promoters that are commonly used in high performance heat exchangers could also severely degrade Type II fluid.
Thermal degradation of Type II fluid can occur if the fluid is stored at high temperature. For instance, the aforementioned widely used Type II fluid must not be stored over 158 degrees F. for any extended period of time. The fluid also degrades when exposed to surfaces at temperatures above 248 degrees F. Under these conditions, the long chain polymer "plates out" on the hot surface, causing the fluid to degrade, and interfering with heat transfer from the hot surface to the fluid. These thermal degradation problems make it difficult to design a system for heating the fluid for use in a deicer. Because Type II fluid has superior anti-icing properties, some airlines use cold "neat" Type II fluid as a part of a two-step deicing process. First the aircraft is deiced using a hot fluid, which might be a mixture of Type II with water, or even a Type I deicing fluid, and then "neat" Type II fluid is applied to achieve anti-icing.
Deicer Tanks
Deicing fluid is generally stored in large tanks which hold many thousands of gallons. The fluid is transferred from the storage tanks into tanks mounted on the deicer vehicles. In many applications, water is also loaded into the tanks on the deicing vehicle. Sometimes the water is loaded into the same tank as the deicing fluid to provide a premixed fluid of a fixed mixture ratio to be sprayed. In other cases, the water is loaded into a separate tank. This makes it possible to spray pure water, pure deicing fluid or any desired mixture of the two.
Fluid Heating
Temperatures
Deicing fluids are expensive, and in varying degrees environmentally hazardous, so it is desirable to deice an aircraft with the least possible amount of fluid. This requires heating the fluid to be sprayed for deicing to a high temperature, on the order of 160 to 180 degrees F. Temperatures above 200 degrees F. are undesirable because excessive steam obscures the deicing operation. Temperatures below 160 degree F. require too much deicing fluid.
Heat Sources
Several different heat sources can be used to heat deicing fluid from the storage temperature to the spraying temperature. These include direct fired heaters, electric heaters and heat derived from an internal combustion engine. Large deicers usually use direct fired heaters burning gasoline or diesel fuel to heat the deicing fluid.
Slow Heating Of Premixed Fluids
Deicers which use direct fired heaters transfer heat from the combustion products to the deicing fluid by means of a heat exchanger. The deicing fluid is premixed in the tank. It is drawn from the tank and pumped through the direct fired heater heat exchanger and then returned to the tank. As the fluid circulates from the tank to the heater and then back to the tank, the temperature of the fluid stored in the tank increases. When the heater discharge temperature (or the stored fluid temperature itself) reaches the desired spraying temperature, the deicing fluid is ready to be sprayed onto the aircraft. The heat release rate of most current burners, as compared to the size of typical deicer tanks, normally result in a tank heat-up time on the order of 45 minutes or more. This process does not produce rapid heating and delivery of the deicing fluid mixture.
Slow Heating Of Separate Fluids
Current deicers also have systems in which the water and deicing fluids are not premixed. In this case, there are deicers in which water is heated in a direct fired burner and then circulated through the a heat exchanger placed in the deicing fluid tank. The entire water flow is passed through the deicing fluid tank for a period of about 45 minutes in order to heat the deicing fluid to the desired temperature for spraying. Then all of the water flow is diverted to a mixer where it is mixed with the preheated deicing fluid from the deicing fluid tank. This process is not suitable for rapid heating and delivery of deicing fluid mixtures.
Rapid Heating
There is now increasing interest in reducing the time needed to heat the deicing fluid to the spraying temperature. One method of current interest is to use a heater with a heat output sufficient to heat the fluid from the storage temperature to the desired spraying temperature in a single pass. This eliminates the time required to heat the entire tank, but it requires a heater with much higher heat output than is required for the recirculation type of heating.
Several organizations, however, have now developed heaters large enough to provide the heat necessary for single pass heating. The current technology of rapid heating is limited to heating pure water or a mixture of pure water and deicing fluid which have been mixed in the tank prior to heating. Passing the fluid through a direct fired heater is suitable for water and Type I fluids, but it may not be suitable for heating Type II fluids. The repeated pumping of the fluid from the tank through the heater and then back to the tank in a recirculating system can cause thermal and/or mechanical degradation of the fluid. The fluid can be degraded by exposure to the high temperature surfaces of the heat exchanger. Direct fired heaters generally have combustion products at temperatures of between 1000 degrees F. and 3500 degrees F. It is difficult to avoid excessive wall temperatures in the heat exchanger tubing used to heat the fluid in a direct fired system. Direct fired heater exchangers usually require high fluid velocities and turbulent flow within the heat exchanger tubing to achieve the compact heat exchanger size required for a practical system. High velocity and turbulent flow cause Type II fluid degradation. Prior to the present invention, there have been no successful method of rapid heating of variable proportions of water and deicing fluids, and no successful method for rapid heating of Type II deicing fluid and/or variable mixtures of Type II fluid with water.